Petrogenesis of high-alumina basalts in South China Sea: Implications for magmatic processes associated with the opening of an oceanic basin

International Ocean Discovery Program (IODP) Expeditions on the northern continental margin of the South China Sea (SCS) have nullified the early notion that the SCS is a magma-poor margin. However, there are continuing debates on how the rapid transition from continent to ocean is supported by geochemical and petrologic data, and whether such a process was related to plate subduction or mantle plume activities. Here we present the bulk-rock and plagioclase phenocryst geochemistry of basalts generated during the early spreading (initial and steady ocean) stage of SCS basin extension, which were drilled at IODP Sites U1500 and U1503, respectively. Combined with published data from the late spreading stage of the basin, the new measurements provide an excellent opportunity to examine the magmatic processes associated with the evolution of the SCS basin. Our results reveal that SCS basalts generally exhibit higher Al contents than global mid-ocean ridge basalts, particularly at lower MgO contents, similar to the characteristics of modern arc basalts. Nevertheless, their origins are site-specific and complex. Some U1503 basalts display strong subduction signals in terms of trace elements, and their correlations with Al₂O₃ content suggest that they are products of partial melting of the mantle wedge, closely related to the northward subduction of proto-SCS. This subduction had a significant effect on triggering the opening of the SCS, rather than by a mantle plume. Basalts from the other sites, including Site U1500, exhibit significant accumulation of plagioclase. Moreover, the An values of the plagioclase in Site U1500 basalts increase with the increase of host magma Al contents, indicating that the floatation mechanism cannot account for plagioclase accumulation. Therefore, we propose that the high abundance of plagioclase in Site U1500 basalts requires rapid ascent of magma, supporting a rapid rifting and strong magmatism during the initial opening of the SCS.